Alarm systems in which messages are transferred via radio offer many advantages over wired networks. In particular the effort of installing these types of network is saved and the individual subscribers can be flexibly arranged and moved around at any locations. The alarm systems include alarm sensors as subscribers or ancillary stations which, in the event of danger being detected, e.g. fire or intrusion, communicate via a radio connection with a central system in which further measures are initiated to remedy the danger, such as signaling alarms to the fire service or the police.
An alarm system is known from WO 92/22883 in which the battery-operated fire alarm and intruder alarm use a radio link to transmit their address, the data measured by the fire or intrusion detector as well as data about the remaining energy of the batteries, to a central system. Such unidirectional systems, although low-cost designs, are however only suitable for systems where the risks are low. To save energy, detectors in such systems are only equipped with one transmitter and only report at infrequent intervals, for example every 24 hours, to the central system. They are not in a position to explicitly search for free radio channels in order to guarantee transmission even if a radio channel is busy.
Since the detectors cannot receive acknowledgements, error-free transport of information cannot be ensured.
Compared to such unidirectional systems, it is possible to significantly increase the security of transmission in radio alarm systems. However a receiver section increases the costs, the unit itself is larger and consumes more energy than a pure transmitter. An alarm system is known from EP 911 775 for example which is a bidirectional system and for which the components are designed to save energy. Faults in this type of system can be securely detected in less than 100 seconds; however what are known as fading holes can lead to unnecessary fault messages. Additional mechanisms are needed to resolve the problem.
Where each subscriber has to establish direct connection with the central system a relatively high transmit power is required for individual subscribers. Therefore in terms of energy consumption what are known as routing systems are better in which subscribers further away can use other subscribers as routers, referred to as routers to make the radio connection to the central system. Thus a method for radio transmission in an alarm system is known from EP 833 288 in which the measured data of an alarm sensor can be transmitted via other alarm sensors as routers to the central system if there is no direct connection to the central system because the radio coverage is too small or if it is disturbed in some way. A time variant hierarchical connection structure between the alarm sensors is provided for this.
To be able to react in a flexible way in such bidirectional systems with flexible routing options to faults in radio traffic and changes in the subscriber structure and to always ensure secure connections it is necessary in such a system for the fact that a subscriber belongs to this system to be known to all other subscribers. This would be possible by using the serial number of a detector as an identification which is known to all other subscribers so that it only allows the known subscribers to take part in data traffic. The disadvantage here however is that such a serial number is relatively long and therefore takes up relatively large amounts of space in the individual subscribers. In addition each time that a new subscriber registers their identification or serial number must be notified to all the others, which takes a very long time and places a significant load on the radio channel. If in addition, the registration of a detector is also to be possible via a number of subscribers the cell identification, that is the serial number, must be entered beforehand since the radio cell information of subscribers not yet registered cannot be transmitted. The setting necessary for this can only be made manually and is therefore relatively prone to error. Such errors in the identification number can also barely be recognized during commissioning.